1. Field of the Invention
The present invention relates to RF receiver/transmitter systems, and more particularly to a zero loss bias "T" for use in a receiver front end.
2. Description of Related Art
The initial stage of a radio receiver/transmitter is referred to as the "front end," and serves to receive RF signals and down-convert them to intermediate frequency (IF) signals. The radio receiver/transmitter front end may include a bias "T", a bandpass filter, an amplifier, and a mixer. A received RF signal passes through the bias "T" to isolate it from a DC signal coupled from a DC power supply. The received signal is then provided to a bandpass filter that rejects adjacent extraneous frequencies outside the bandwidth of the received signal. The RF amplifier amplifies the received and filtered signal to a desired amplitude level. The mixer multiplies the amplified signal with a locally generated signal to produce an intermediate frequency (IF) or baseband received signal corresponding to a difference in frequency between the received signal and the locally generated signal. Thereafter, the IF or baseband received signal is provided to downstream demodulation stages that recover the information contained within the originally received signal.
More particularly, the bias "T" provides selective electrical isolation of a DC power supply node from RF signals present in the circuit and allows DC currents to flow with little or no restriction while impeding the flow of RF currents. A conventional bias "T" is disclosed in U.S. Pat. No. 5,105,172 to Khatibzadeh, et al. A bias "T" is generally used in RF designs to couple a DC voltage onto a line used for AC (RF) signals, or to detect/remove the DC component of a composite signal. For example, the bias "T" may be used to couple a switchable DC voltage out of an antenna port of a radio in order to accomplish both switched gain antenna selection and diversity antenna selection.
Many methods exist for coupling DC voltages onto AC signal lines. In its simplest form, a bias "T" may be provided by an inductive choke as disclosed in U.S. Pat. No. 5,838,215 to Gu, et al. For frequencies below 1 GHz, it is known to construct a bias "T" using discrete electrical components. Above 1 GHz, however, a bias "T" is usually constructed of tuned elements on a circuit board (such as in microstrip or stripline designs) and may or may not contain additional discrete passive components. A problem with the bias "T" designs used for high frequency applications is that they permit frequencies outside the desired band to pass. Another problem with these bias "T" designs is that they cause a loss of the desired AC signal.
Typical bandpass filters used in receiver front ends also cause a certain amount of loss to the AC signal. These filters are generally of the "monolithic" type and are provided from a component supplier as a single, non-tunable unit. Even though monolithic filters have an associated loss, the extent of the loss is likely to be less than a corresponding filter unit made from discrete components. Therefore, monolithic filters are desirable notwithstanding their associated signal loss. Multiple types of monolithic filters are commercially available. The most common ones are ceramic and surface acoustic wave (SAW) resonator filters as disclosed in U.S. Pat. No. 5,666,092 to Yamamoto, and inductance capacitance (LC) filters as disclosed in U.S. Pat. No. 4,698,605 to Imamura, et al.
Ceramic and SAW filters each use the same general principle. They are also known as "interdigital" filters and consist of several metallic fingers of carefully calculated geometry on a substrate. Energy at certain frequencies is resonated by respective ones of these fingers. The energy is transferred from finger to finger causing the desired frequencies to pass through the device, thereby providing a bandpass filter. A general characteristic of ceramic and SAW filters is that they do not include a DC path from input or output to ground. LC filters are combinations of inductors and capacitors that are made very small on a substrate and are combined in a single small package. Unlike ceramic and SAW filters, LC filters contain one or more inductors internally that do provide a DC path to ground from the input or output terminals.
In summary, the receiver front end circuits incur losses, which are extremely critical to receiver performance. The losses will directly subtract from the receiver's noise figure and hence the receiver's sensitivity and performance. Accordingly, the need exists for a method to minimize or eliminate the losses in standard bias applications while passing only frequencies in the desired passband.